Optical strain sensing is a technology useful for measuring physical deformation of a waveguide caused by, for example, the change in tension, compression, or temperature of an optical fiber. A multi-core optical fiber is composed of several independent waveguides embedded within a single fiber. A continuous measure of strain along the length of a core can be derived by interpreting the optical response of the core using swept wavelength interferometry. With knowledge of the relative positions of the cores along the length of the fiber, these independent strain signals may be combined to gain a measure of the strain profile applied to the multi-core optical fiber. The strain profile of the fiber refers to the measure of applied bend strain, twist strain, and/or axial strain along the length of the fiber at a high (e.g., less than 50 micrometers) sample resolution.
Previous patents have described shape sensing with multi-core optical fibers (e.g., see U.S. Pat. Nos. 7,781,724 and 8,773,650 incorporated by reference). Some applications for shape sensing fiber require a high degree of confidence in terms of the accuracy and reliability of the shape sensing output. A non-limiting example application is robotic arms used in surgical or other environments.
In performing position and shape sensing measurements of the fiber, accuracy is limited by how well the strain signals from the independent optical cores can be recombined. Further, if accuracy levels are required that are for example on the order of 0.1% of the length of the measurement fiber, compensation for these variations is important. An ideal structure for this purpose occurs when the core waveguides are located exactly as specified by the design of the fiber after manufacture, and the physical properties of the cores are identical. But in practice, manufacturing processes are not capable of producing an ideal fiber structure. Therefore, variations, such as in core location, length, and index of refraction are observed in actual fiber structures. Unfortunately, variations from an ideal fiber structure cannot be physically measured to this desired degree of accuracy by known techniques.
In order to calculate the shape of a multi-core fiber, certain things must be known about the fiber including for example the core locations (referred to as “core geometry” and expressed as a radius and an angle), the spin rate of the fiber, and the group index of the cores. Determining these parameters through various measurement techniques is referred to as calibrating the fiber or fiber sensor. Known methods for fiber sensor calibration are described in U.S. Pat. Nos. 8,531,655 and 8,773,650, which are incorporated by reference.
One drawback with known sensor calibration techniques is that they are manual techniques which are time-intensive and expensive. Another area for improvement is that known sensor calibration techniques are not suited to a manufacturing environment because they do not account for all of the possible variations in the fiber properties that are possible in fiber manufacture. For example, previous calibration techniques did not account for variations in fiber diameter along the length of a sensor, the effect of spin rate variations on twist sensitivity, variations in the strain optic coefficients core-to-core, or variations in the thermo-optic coefficients core to core. In addition, known calibration and/or measurement techniques do not account for the effects of tension on the fiber. New calibration technology is needed to more completely calibrate a sensor and to do so in an automatable way.